This invention relates to devices and systems for detecting bond degradation and more particularly, to micro-electronic devices and systems for detecting degradation in an adhesive bond formed between two or more structures.
Bonded structures that are comprised of materials adhesively fixed together can provide relatively light weight with great strength. An adhesive material is typically applied to one or both of the materials, which are then pressed into contact. After the adhesive cures, the adhesive material holds the two materials together, without physically altering the state or condition of the respective materials. Bonded materials offers many advantages over traditional joined materials, which usually require drilling and riveting of plates with the consequent problems associated with increased stress at the rivet holes. Thus, the bonded materials do not loose any of their inherent strength or other features of the materials themselves. Bonded materials are extensively used, for example, in the aircraft industry for making aircraft wings, fuselage coverings, and other body structures.
A complete release of the bond between materials, called a disbanding, is unusual and, fortunately, is typically rather easy to detect. One problem with bonding technology is reliably detecting a loss of strength of the bonded structure, as opposed to a complete disbonding of parts of the structure. The lack of a test for partial bond degradation has (1) limited the application of bonded manufacture and repair technology, especially in critical components of air-frames; (2) meant that the current technology relies heavily on good bonding practice and procedures performed by trained technicians; (3) resulted in some aircraft in-service bonded repair failures with consequent impact damage with the separating bonded repair patch; and (4) resulted in unnecessary, and expensive, removal of old or suspect bonded repair patches for replacement with a new patch.
There is a great need for a reliable technique for monitoring the condition of bonded components in both military and civilian applications. The development of such techniques would lead to a greater confidence and hence wider application of bonding technology for manufacture and repair, more reliable condition-based maintenance of bonded structures, and improvements in bonding methods through the understanding gained from the use of degradation-sensitive sensors in the study of bond degradation mechanisms.
Macroscopic Fracture Mechanics Testing
Macroscopic examination of fractured surfaces has revealed that adhesive failure can occur at the adhesive bond interface between materials. Corrosion has been found to exist in the metal layers that are bonded together, under some conditions. Generally, fracture surfaces of specimens subjected to dry air show that failure occurs principally in the adhesive film itself. Fracture analysis of specimens tested in humid air, however, reveal large areas of adhesive failure at the bond interface, and also reveal evidence of corrosion in the bonded materials. This clearly demonstrates the need for a reliable metal interface sensor.
Common macroscopic testing methods that will be known to those skilled in the art include the xe2x80x9cBoeing wedge testxe2x80x9d (ASTM D3762-79, reapproved 1988) and the Constant Load Point Displacement Rate Test.
The Boeing wedge test is the standard test method for adhesive bonded surface durability of Aluminum (Al). The method simulates in a qualitative manner the forces and effects on any adhesive bond joint at a metal adhesive/primer interface. In the Boeing wedge test, one end of a double-cantilever beam (DCB) specimen is opened to a specified and constant crack opening displacement using a wedge. The elastic energy release rate depends on the crack length. The crack length typically cannot be measured accurately due to the uncertainty in locating the crack tip. Hence there is a need for a test that eliminates the need to measure the crack length.
The Constant Load Point Displacement Rate Test (CDRT) eliminates the need to measure the crack length. A bonded DCB specimen is opened at a constant load point displacement rate. The rate selected strongly influences the crack velocity. In humid air, the load-point displacement rate can be selected to drive the crack at a velocity that is either faster or slower than the rate of degradation of the bond ahead of the crack tip. The application of fracture mechanics to the CDRT leads to expressions for both the elastic energy release rate and crack velocity.
Unfortunately, none of the methods of fracture mechanics applied to the Boeing wedge test lead to well-defined parameters describing the rate of bond degradation. By contrast, the CDRT reveals the range in crack velocity where crack propagation undergoes a transition from un-degraded to degraded material. The transitional crack velocity is directly linked to the rate of bond degradation under apparent constant stress intensity at the crack tip. A limitation of the CDRT method is the long times typically required to obtain data from a range of load point displacement rates (up to 3 months).
Microscopic Investigation
Scanning Electron Microscopy (SEM) techniques have been used to investigate thin adhesive layers in adhesive/metal interfaces. The manner in which water degrades an adhesive bond is complex. It is believed that the processes of bond degradation involve the migration of water ahead of the crack tip, the decoupling of the adhesive or coupling agent from the oxide, and the hydration of oxide at the unprotected sites on the oxide surface. Models of the x-ray generation close to an interface, using Monte Carlo and finite element techniques, allow for the determination of thin layers on the interface and hence microscopic insight of the adhesive/metal interface. The SEM observations imply that the metal oxide is not being degraded by diffusion of water through the bulk oxide. Thus diffusion of water must be occurring either along the epoxy/metal oxide bond interface or within the epoxy. The macroscopic fracture mechanics tests would indicate that the diffusion of water is suspected to be along or very close to the interface.
Electrical impedance measurements from 10 Hz to 10 MHz have been performed on bonded specimens after different times of exposure to water. The results show that the metal-epoxy-metal bond initially behaves like a capacitor and then becomes more resistive at lower frequencies with longer exposure to water. This resistive behavior is due to the water penetrating the adhesive. Ions in the water oscillate under the influence of the alternating electric field. Above a certain frequency these ions will not be able to respond to the varying electric field, due to inertia, and the bond will once again appear capacitive. The impedance versus frequency dependence clearly differentiates between a bond with few voids and a bond with many voids after exposure to water. These measurements are also sensitive to the continuing degradation of the bond with ingress of water. Development of this technique could lead to a nondestructive technique for interrogating the integrity of adhesive bonds in the field.
Current NDE Systems for Bonded Components
Three conventional non-destructive evaluation (NDE) techniques are commonly used with adhesively bonded repairs. The three techniques are (1) x-ray tomography, (2) ultrasonics (including laser ultrasonics), and (3) infra-red imaging. All of these techniques are external to the bonded component. All are sensitive to gross disbonding and corrosion and, with infra-red imaging, can detect voids larger than approximately 1 square millimeter (mm) in a bond. None of the three techniques are sensitive to reduced bond strength when there is still good physical contact between the two bonded surfaces (the so called xe2x80x9ckissing bondxe2x80x9d problem). All of these NDE techniques require good access to the bonded region visually and/or manually. The x-ray systems often suffer from occupational health and safety considerations, since typical x-ray energies used are around 200 keV.
Honeywell Smart Sensor Technology
An electrochemical-based smart sensor system was developed by Honeywell Technology Center to provide early warning detection of corrosion related symptoms in hidden locations of aircraft structures. The Honeywell system, called xe2x80x9cSmart Aircraft Fastener Evaluation (SAFE)xe2x80x9d system, detects symptoms of extensive pH cycling and the presence of extensive moisture.
The SAFE system measures ionic activity in real time. The SAFE system uses a prototype Lawrence Livermore National Laboratory (LLNL) multi-element micro sensor array developed under the Smart Metallic Structures (SMS) program designed to sense a specific ionic phenomenon. Those skilled in the art will be familiar with the SAFE system and LLNL sensors.
The sensor system is packaged in Hi-Lok fasteners used to bolt aircraft skin together. Various methods for mounting the sensors in the fasteners have been used. Typically, a corrosive electrolyte solution is transported from between aircraft skin layers via a series of capillary channels located in the well of the fastener to an internally-located environmental chamber, where the LLNL sensor array can sense the effects and quantify early corrosion symptoms. Also used are designs with the sensors mounted in a shallow slot on the outside of the fastener.
The SAFE system provides sufficient corrosion monitoring capability in bolted areas. Unfortunately, corrosion detection using the SAFE system is often times only possible through empirical association without physical insight as to the cause of the bond degradation. Further disadvantageously, the hollow structure of the Hi-Lok fastener tends to lower the structural strength of the repaired structure. Further, the SAFE system sensors rely on the transport of ions through capillaries which can clog. Finally, the size of the sensors used in the SAFE system precludes their use in structural repairs requiring small bond lines such as aircraft and the like.
What is therefore needed is a bond degradation sensor which can detect the presence and rate of bond degradation in an adhesive bond line formed between two or more structures. Further needed is a bond degradation sensor of minimal size to permit the sensor""s implementation within a thin adhesive bond line.
The present invention provides for a micro-electronic bond degradation sensor which is operable to detect the presence and rate of bond degradation in an adhesive bond line formed between two or more structures. The micro-electronic bond degradation sensor can be fabricated on the order of 20-100 xcexcm thick and can easily be embedded into very thin adhesive bond lines to provide accurate bond monitoring capability.
In one embodiment, the micro-electronic bond degradation sensor includes a sensor substrate having a sensor stud and a power stud extending therefrom. A sensor circuitry is additionally formed on the sensor substrate. The sensor circuitry includes a voltage-to-current amplifier having an input coupled to sensor stud and an output coupled to the power stud. The voltage-to-current amplifier is operable to convert a voltage signal occurring along the sensor stud to a current signal output along the power stud.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, which illustrates, by way of example, the principles of the invention.